A combined acousto-optic modulation method based on precise diffraction superposition

Abstract

The present invention belongs to the field of laser control technology, specifically a combined acousto-optic modulation method based on precise diffraction superposition. In the process of realizing the coherent superposition of the Bragg diffraction amplitudes of two acousto-optic modulators by using a 4‑F optical system, the present invention moves one acousto-optic modulator to an appropriate distance from the imaging position of another acousto-optic modulator, suppresses the diffraction phase shift broadening of the Bragg diffraction of the focused light beam by momentum echo, and adjusts the high-order diffraction phase shift at the same time, and accurately suppresses the high-order diffraction loss by the coherent destructive effect, thereby obtaining the above diffraction efficiency and the above single-mode suppression ratio of the transmitted light; the present invention can also greatly reduce the power requirement of the radio frequency signal used to drive the acousto-optic modulation, expand the selection of acousto-optic modulation crystals, and help develop low insertion loss, high damage threshold acousto-optic modulation devices and high-frequency acousto-optic modulation devices. The present invention has broad application prospects in the fields of pulsed laser modulation, optical and quantum information processing, laser coherent beam splitting / combining, and other precision optical manipulation fields.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser control, and in particular relates to a combined acousto-optic modulation method utilizing precise diffraction superposition to improve control efficiency and bandwidth. Background Art

[0002] From basic scientific research to industrial control, laser applications require on-demand modulation of laser output. Acousto-optic modulation (AOM)[1] uses radio frequency signals to control the crystal acoustic field to achieve Bragg diffraction of the incident light, thereby achieving precise adjustment of the power, phase, frequency, and propagation direction of the diffracted beam. S Often in thousands of meters per second, for beams with width w below a millimeter, the acousto-optic control bandwidth Δω M ≈v S / w can be as high as tens of MHz, and the control time τ M =1 / Δω M It can be as low as ten nanoseconds. Compared with the faster electro-optical modulation [2], acousto-optic modulation has significant advantages in terms of multi-degree-of-freedom control, precise modulation phase and amplitude, wide range of laser wavelength and power selection, and low instrument drive requirements. The combination of these advantages and high-speed modulation capabilities makes acousto-optic modulation devices have an almost irreplaceable important technical position in basic research and laser technology development. However, according to research, conventional acousto-optic modulation technology [1,3] has at least the following shortcomings:

[0003] (1) The efficiency of acousto-optic diffraction is low. 0 Power input, the maximum diffraction efficiency of conventional AOM η = P 1 / P 0 Generally, it does not exceed 90%. Such efficiency limits the application of AOM in fields with strict efficiency requirements, such as high-frequency iteration[3], laser power synthesis[4], and optical quantum information processing[5].

[0004] (2) Corresponding to the low diffraction efficiency is the significant residual zero-order after the AOM is turned on. Therefore, ordinary AOM cannot freely split the incident light into zero-order and first-order coherent beams and realize the function of high-contrast optical beam splitters. High-speed controllable optical beam splitters have important application prospects. For example, they can be used to realize controllable routing of pulsed lasers, or to construct two-way interferometry to control the group velocity of pulsed lasers by beam splitting ratio [2].

[0005] (3) For ordinary acousto-optic modulation, obtaining a higher diffraction efficiency η often requires a large incident light spot w, which sacrifices the modulation bandwidth δω. M At the expense of: On the other hand, in order to obtain a modulation bandwidth of tens of megahertz, the usual practice is to focus the incident light spot waist w to less than 100 microns. However, the wave vector of the focused light is broadened, destroying the Bragg condition and reducing the diffraction efficiency η.

[0006] (4) The driving RF power of ordinary acousto-optic modulation is relatively high, which not only limits the development of high-frequency modulation technology, but also limits the selection of acousto-optic crystals and acoustic field design. For example, quartz crystals with excellent optical quality and low price often need to provide nearly ten watts of RF driving power. The large power consumption brings many design restrictions. Acousto-optic modulation often uses crystals such as TeO2[6]. Although the driving power requirement is slightly lower, the optical insertion loss is large and the optical damage threshold is low, making it difficult to use for high-power lasers.

[0007] (5) In the combined acousto-optic modulation technology [6,7] related to the present invention, the authors proposed to achieve higher diffraction efficiency and control bandwidth through precise imaging with N>1 acousto-optic modulators. However, for the configuration with N=2, the diffraction efficiency η is still difficult to exceed 95%. Although increasing the number of AOMs N can continuously improve the diffraction efficiency, the problems that follow are resource consumption and increased insertion loss. Summary of the invention

[0008] The purpose of the present invention is to provide an ultra-high efficiency combined acousto-optic modulation method based on precise diffraction superposition to suppress high-order diffraction losses, thereby obtaining an ultra-high diffraction efficiency of more than 99% and a transmitted light single-mode suppression ratio of more than 30dB.

[0009] The combined acousto-optic modulation method based on precise diffraction superposition provided by the present invention first constructs a coding module capable of synchronizing radio frequency signals and a coherent combined acousto-optic modulation system capable of precise adjustment; wherein:

[0010] The encoding module capable of synchronizing radio frequency signals provides a phase-stable and arbitrarily programmable radio frequency signal, which is amplified by a signal amplifier and then input into a coherent combined acousto-optic modulation system to drive the acousto-optic modulator AOM in the system to work;

[0011] The coherent combined acousto-optic modulation system is composed of two acousto-optic modulators AOM of the same model with similar geometric dimensions and a double-lens 4-F optical imaging system; wherein the acousto-optic modulator converts the radio frequency signal into a sound wave of corresponding frequency, intensity and phase, and produces acousto-optic diffraction on the incident pulsed laser; the double-lens 4-F optical imaging system is composed of two aplanatic lenses with a focal length of F; the first (i.e., the previous) acousto-optic modulator AOM 1 The diffracted output is precisely imaged with a magnification of M = 1 to the second (i.e. the next) acousto-optic modulator AOM that propagates in the same direction as the sound wave. 2 , forming interference of two diffractions.

[0012] In the process of realizing the coherent superposition of the Bragg diffraction amplitudes of two AOMs using a dual-lens 4-F optical imaging system, the second AOM 2 Remove the first AOM 1The imaging position is at an appropriate distance, the diffraction phase shift broadening of the focused beam Bragg diffraction is suppressed by momentum echo, and the high-order diffraction phase shift is adjusted at the same time, and the high-order diffraction loss is accurately suppressed by the coherent destructive effect, so as to obtain a diffraction efficiency of more than 99% and a single-mode suppression ratio of more than 30dB for transmitted light; the specific steps are as follows:

[0013] (1) Let the length of the AOM crystal be L; use the first AOM (AOM 1 ) for a beam with a central wave vector located at The laser with a transverse wave vector broadening of Δk is subjected to single-frequency acousto-optic modulation (where the wave vector of the normal incident beam is denoted by k). 0 , the modulated sound field wave vector is denoted as k s ) The center of the output wave vector corresponding to the m-order Bragg diffraction is located at There is a wave vector broadening Δk=π / w, where w is the beam waist of Gaussian light.

[0014] (2) Using a dual-lens 4-F optical imaging system, the first acousto-optic modulator (AOM) 1 ) The crystal acoustic field and the light beam interact at the object plane, which is precisely imaged to the second acousto-optic modulator AOM. 2 Center; Second Acousto-Optic Modulator AOM 2 The direction of the sound field and the first acousto-optic modulator AOM 1 The direction of the image of the sound field remains the same.

[0015] (3) Use weak RF signals to drive the AOMs separately 1 and AOM 2 , fine-tune the incident light angle so that the AOM 1 and AOM 2 The m=1 order diffraction efficiency η<50% and is close to being the same; for a commercial AOM designed and optimized at 100% RF drive, when m≠0, the total diffraction loss of the 1st order is at the level of 1%.

[0016] (4) Using AOM 2 About AOM 1 All levels of diffracted light emitted are diffracted twice, changing the AOM 2 The driving phase makes the m=1st order diffraction brightest, and the AOM is fine-tuned forward and backward along the optical axis of the dual-lens 4-F optical imaging system. 2 Position δL, it is found that the m=-1,2 order diffraction intensity has The spatial period oscillation of δL is n =nδL h +δL off The local minimization of the m = -1,2 order diffraction loss can be achieved at . Where n is the oscillation period number, n = 0, 1, 2..., δL offis the bias, and its value depends on the acoustic field length, frequency, and other acoustic field distribution details of the acousto-optic modulator (AOM).

[0017] (5) Joint optimization of AOM through program control 1 and AOM 2 The driving power, The global optimal drive configuration is obtained at . Here […] is the integer symbol, is the average refractive index of the acousto-optic crystal, is the optimal distance for momentum echo compensation for Δk diffraction phase broadening under the two-mode approximation ignoring high-order diffraction losses. The coefficient ξ≈0.2 is related to the sound field distribution profile. When AOM 2 The position of δL opt When the combined acoustic-optical modulation has a lower requirement on the Bragg condition due to momentum echo compensation, the zero-order residual is suppressed. On the other hand, this distance ensures that the AOM 1 and AOM 2 The higher-order diffraction caused by this is destructive. opt The distance can achieve m = +1 order diffraction efficiency η opt >99%, m=0 level single-mode suppression ratio greater than 30dB target.

[0018] (6) Further fine-tuning the driving sound field frequency ω S =v S k S , the above step (5) can be The bias δL in off Reduce to zero, while achieving high-order diffraction suppression and momentum echo optimization, to obtain a higher diffraction efficiency η opt .

[0019] (7) During the above steps (4) and (5), the output is formed by stable multi-path interference with a certain phase relationship; the optimal output of the diffraction order m=1 is generated by the first acousto-optic modulator AOM 1 and the second AOM 2 The phase difference corresponding to the driving RF signal and the offset distance δL of the 4-F optical imaging system are determined by changing the first acousto-optic modulator AOM 1 and the second AOM 2 Common drive phase While maintaining the phase difference The precise phase control of the diffracted light can be achieved by changing the first acousto-optic modulator AOM. 1 and the second AOM 2 The driving strength can achieve a diffraction efficiency η from zero to η optFine adjustment of the optical beam splitter and optical routing functions can be achieved.

[0020] The method of the present invention has a very high diffraction efficiency (η opt >99%) and zero-order suppression ratio (single-mode coupling suppression rate α>30dB), with extremely high working bandwidth and precise phase modulation capability, it can replace traditional technologies such as piezoelectric, electro-optical, and traditional acousto-optic modulation in the following technical fields, update combined acousto-optic modulation technology [6,7], and improve the speed, efficiency, accuracy, and light intensity damage threshold of laser modulation:

[0021] (1) Pulse optical routing technology: Realize time division multiplexing (TDM) of pulse lasers[7] and quantum light sources[4].

[0022] (2) Pulse optical path coherent accumulation technology: multiple laser beams are coherently combined to form a higher power laser[5].

[0023] (3) The group velocity of the pulsed laser is adjusted by the interferometer splitting ratio to achieve carrier-envelope phase regulation of the high-power optical comb [2].

[0024] (4) Through the iterative application of efficient acousto-optic modulation, large-scale controllable movement of optical frequencies at the level of tens of GHz can be achieved [3].

[0025] Furthermore, the present invention is not limited to the coherent combination of two AOMs. The coherent combination of multiple AOMs can be realized by using a multi-lens 4-F optical imaging system. Assuming that the number of sub-AOMs in the combined acousto-optic modulation system is N, the driving power required for each sub-AOM is reduced to the traditional requirement. Therefore, the present invention can significantly reduce the demand for radio frequency drive power of acousto-optic modulation, thereby expanding the selection range of acousto-optic modulation crystals. For example, it is convenient to use low acousto-optic efficiency materials such as quartz crystals to develop acousto-optic modulation systems with low insertion loss and high damage threshold, as well as to develop high-frequency modulation, deep ultraviolet wavelength laser modulation and other technologies; in addition, in the N-AOM system, by fine-tuning the corresponding δL displacement of the acousto-optic modulator (AOM) in each imaging system, the diffraction efficiency can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the dual-AOM coherent combination acousto-optic modulation system of the present invention.

[0027] Figure 2 The diffraction results of the dual-AOM coherent combination system under different driving conditions are shown.

[0028] Figure 3 The figure shows the change of the m=1 order diffraction efficiency when the dual-AOM coherent combination system is driven by RF of different intensities, where the horizontal axis is the ratio of the actual RF driving intensity to the optimized driving intensity.

[0029] Figure 4 The changes in the maximum diffraction efficiency of the dual-AOM coherent combination system and the single AOM and the corresponding 0th-order suppression ratio when the driving frequency is changed are demonstrated.

[0030] Figure 5 The AOM in the dual AOM coherent combination acousto-optic modulation system is given by numerical simulation. 2 The relationship between the position shift δL and the diffraction efficiency of m=-1,0,1,2. High-order diffraction (m=-1,2) shows periodic oscillation, and the oscillation period is DETAILED DESCRIPTION

[0031] The present invention uses the method of precise diffraction superposition to achieve ultra-high acousto-optic diffraction efficiency far exceeding that of the traditional single AOM. The core of this method is to use the AOM in the process of precise diffraction coherent superposition. 2 Move out of AOM 1 The imaging position is at an appropriate distance, and the diffraction phase shift broadening of the focused beam Bragg diffraction is suppressed in the form of momentum echo. At the same time, the high-order diffraction phase shift can be adjusted to accurately suppress the high-order diffraction loss by the coherent destructive effect, thereby obtaining a 1st-order diffraction efficiency of more than 99% and a 0th-order single-mode suppression ratio of more than 30dB.

[0032] by Figure 1 The dual AOM coherent combination system shown in the figure is used as an example to conduct experiments. In the experiment, the two AOMs as basic units are of the same model, with an optimized frequency of 80MHz and a sound speed of 4260m / s. Continuous light (λ = 780nm) is incident on the AOM under Bragg conditions. 1 The outgoing light is accurately imaged to the AOM by a 4-F optical imaging system consisting of two achromatic lenses (F=100mm). 2 To perform secondary diffraction, AOM must be ensured 2 The sound field direction and AOM 1 The direction of the sound field image is the same; change the AOM 2 The driving phase makes the first order diffraction brightest; fine-tune the AOM forward and backward along the optical axis of the 4-F imaging system 2 The final diffraction intensity distribution is collected by a CCD camera and fed back to the optimization program to optimize the AOM. 1,2 The driving power of the device can finally achieve excellent diffraction effect, such as Figure 2 (c) as shown.

[0033] Figure 2The display shows the output results of the above demonstration experimental device under three different driving conditions: (a) is the state when both AOMs are not driven, and no diffraction occurs at this time; (b) is the state when only one AOM is driven to reach the diffraction maximum, at which time the first-order diffraction efficiency is about 90%, and high-order diffraction spots can be clearly seen; (c) is the state when both AOMs are driven simultaneously under the optimal driving conditions, at which time the first-order diffraction efficiency is >99%, and high-order diffraction spots are almost invisible, and the 0th-order spot is also greatly suppressed. Figure 4 It can be seen that in free space, the 0th-order suppression ratio can reach about 22dB. After mode selection of single-mode light, the 0th-order suppression ratio can be further increased to more than 30dB.

[0034] Figure 3 The experimental results show that the first-order diffraction efficiency of the dual-AOM coherent combination system can be precisely controlled by changing the AOM drive intensity without changing the two AOM drive phase difference.

[0035] Figure 4 The figure shows the maximum 1st-order diffraction efficiency (solid line) and the corresponding 0th-order suppression ratio (dashed line) in free space that can be achieved by single AOM diffraction and dual AOM coherent combination diffraction at other driving frequencies. As the driving frequency continues to shift from the optimized frequency (80MHz), the maximum 1st-order diffraction efficiency and 0th-order suppression ratio of the two diffraction modes are constantly decreasing, but within the driving range of 80±40MHz, the effect of dual AOM coherent combination diffraction is always better than that of single AOM diffraction. The experimental results show that the dual AOM coherent combination system not only has better 1st-order diffraction efficiency and 0th-order suppression ratio than a single AOM, but also has a larger driving frequency bandwidth than a single AOM.

[0036] Figure 5 The figure shows the influence of the fine-tuning distance δL on different diffraction levels based on numerical simulation. The horizontal axis is the fine-tuning distance δL, and the vertical axis is the diffraction efficiency. The trend change between the 1st and 0th order efficiencies is affected by the diffraction phase shift broadening of the focused beam Bragg diffraction. When δL can meet the optimal distance for momentum echo compensation When ξ≈0.2, the first-order diffraction efficiency reaches its peak. The oscillation trend in a small range is affected by the high-order diffraction. As δL changes continuously, the coherence conditions of the high-order diffraction in the secondary diffraction change periodically, and the high-order diffraction also oscillates periodically, with a period of The numerical simulation results show that the fine-tuning distance of the second AOM in the dual-AOM coherent system is the combined effect of the momentum echo compensation distance and the corresponding position of the lowest point of the high-order oscillation.

[0037] References:

[0038] [1]J.Thom et al,“Accurate and agiledigital control of optical phase,amplitude and frequency for coherent atomic manipulation of atomic systems,”Opt.Express 21,18712(2013).

[0039] [2]W.Hansel,M.Giunta,M.Fischer,M.Lezius,and R.Holzwarth,Rapidelectro-optic control of the carrierenvelope-offset frequency for ultra-lownoise frequency combs,2017Joint Conference of the European Frequency and TimeForum and IEEE International Frequency Control Symposium,EFTF / IFC 2017-Proceedings,128–129(2017).

[0040] [3]Zhou et al,Laser frequency shift up to 5GHz with a high-efficiency12-pass350-MHz acousto-optic modulator,Rev.Sci.Instrum.91,033201(2020).

[0041] [4]S.Yu,et al,A universal programmable Gaussian boson sampler fordrug discovery,Nature Computational Science 3,839,848(2023).

[0042] [5]Arno Klenke et al,Coherent Beam Combination of Ultrafast FiberLasers,IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS,VOL.24,NO.5,(2018).

[0043] [6] A. Feldman, "Measurement of the Photoelastic Contents of OpticalMaterials," Opt.Eng.17,453(1978).

[0044] [6] Wu Saijun, Ma Yudi, Liu Ruijuan, Qiu Liyang, invention patent: High-bandwidth composite acousto-optic modulation method based on multiple 4F imaging, patent number: CN113777811B.

[0045] [7] Wu Saijun, Liu Ruijuan, Ma Yudi, Qiu Liyang, invention patent: Ultra-high-speed frequency division method of laser pulse repetition rate based on dual-path acousto-optic interference, patent number: CN113725714B.

Claims

1. A combined acousto-optic modulation method based on precise diffraction superposition, characterized in that: First, a coding module that can synchronize RF signals and a coherent combined acousto-optic modulation system that can be precisely adjusted are constructed; The encoding module capable of synchronizing radio frequency signals provides a phase-stable and arbitrarily programmable radio frequency signal, which is amplified by a signal amplifier and then input into a coherent combination acousto-optic modulation system to drive the acousto-optic modulator (AOM) in the system to work; The coherent combined acousto-optic modulation system is composed of two acousto-optic modulators (AOMs) of the same model with similar geometric dimensions and a double-lens 4-F optical imaging system; wherein the acousto-optic modulator converts the radio frequency signal into a sound wave of corresponding frequency, intensity and phase, and generates acousto-optic diffraction on the incident pulsed laser; the double-lens 4-F optical imaging system is composed of two aplanatic lenses with a focal length of F; the diffraction output of the first acousto-optic modulator (AOM1) is accurately imaged to the second acousto-optic modulator (AOM2) with the same propagation direction of the sound wave with a magnification of M=1, so as to form interference of two diffractions; The specific steps of combined acousto-optic modulation are as follows: (1) Let the length of the acousto-optic crystal of the acousto-optic modulator (AOM) be L; use the first acousto-optic modulator (AOM1) to modulate a beam with a central wave vector at The laser with a transverse wave vector width of Δk is subjected to single-frequency acousto-optic modulation, and the center of the output wave vector corresponding to the m-order Bragg diffraction is located at Δk=π / w, w is the beam waist of Gaussian light; k0 is the wave vector of the normal incident beam, k s is the modulated sound field wave vector; (2) Using a dual-lens 4-F optical imaging system, the crystal acoustic field of the first AOM (AOM1) and the center of action of the light beam are used as the object plane, and the image is accurately imaged to the center of the second AOM (AOM2); the direction of the acoustic field of the second AOM (AOM2) is the same as the direction of the image of the acoustic field of the first AOM (AOM1); (3) The first AOM1 and the second AOM2 are driven by a weak RF signal, respectively, and the incident light angle is fine-tuned so that the m=1-order diffraction efficiency η of the first AOM1 and the second AOM2 is less than 50% and is nearly equal; for the optical modulator (AOM) designed and optimized at 100% RF drive, at this time m≠0, the total diffraction loss of the 1st order is at the level of 1%; (4) The second AOM (AOM2) is used to perform secondary diffraction on all levels of diffracted light emitted by the first AOM (AOM1), and the driving phase of the second AOM (AOM2) is changed to make the m=1 level diffraction the brightest. The position of the second AOM (AOM2) is fine-tuned forward and backward along the optical axis of the dual-lens 4-F optical imaging system by δL. When the diffraction intensity of the m=-1 and 2 levels is The spatial periodic oscillation of δL is n =nδL h +δL off Achieve local minimization of m = -1,2 order diffraction loss; where n is the oscillation period number, n = 0, 1, 2…, δL off is bias; (5) The driving power of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) are jointly optimized by program control. The global optimal drive configuration is obtained at , where […] is the integer symbol, is the average refractive index of the acousto-optic crystal, It is the optimal distance for achieving momentum echo compensation for Δk diffraction phase broadening under the two-mode approximation while ignoring high-order diffraction losses. The coefficient ξ is related to the sound field distribution profile.

2. The combined acousto-optic modulation method based on precise diffraction superposition according to claim 1, characterized in that: Further fine-tuning the driving sound field frequency ω S =v S k S , replace step (5) The bias δL in off Reduce to zero, while optimizing high-order diffraction suppression and momentum echo compensation to obtain a higher diffraction efficiency η opt .

3. The combined acousto-optic modulation method based on precise diffraction superposition according to claim 1, characterized in that: During steps (4) and (5), the output is formed by stable multi-path interference and has a certain phase relationship; the optimal output of the diffraction order m=1 is determined by the phase difference corresponding to the RF signal driven by the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) and the offset distance δL of the 4-F optical imaging system; by changing the common driving phase of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2), the output of the diffraction order m=1 is determined by the phase difference corresponding to the RF signal driven by the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) While maintaining the phase difference The diffraction efficiency η can be changed from zero to η by changing the driving strength of the first AOM (AOM1) and the second AOM (AOM2). opt Fine adjustment of the optical beam splitter and optical routing functions can be achieved.

4. The combined acousto-optic modulation method based on precise diffraction superposition according to any one of claims 1 to 3, characterized in that: In the coherent combined acousto-optic modulation system constructed, the number of acousto-optic modulators (AOMs) is N, and the number of its corresponding 4-F optical imaging systems is N, N ≥ 2, then the RF signal power driving a single acousto-optic modulator (AOM) is reduced to the traditional requirement. It can alleviate the pressure of power consumption and expand the selection and design space of acousto-optic crystals. Moreover, it can further improve the diffraction efficiency by fine-tuning the corresponding δL shift of the acousto-optic modulator (AOM) in each imaging system.

Images